n = int(raw_input())
fee = []
for i in range(0, n-1):
	fee.append(int(raw_input()))
b, c = map(int, raw_input().split())
now = c
sum = 0
for i in range(0, n-1):
	sum += fee[i]*now
	b, c = map(int, raw_input().split())
	now = now - b + c
print sum